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Total Synthesis of the Phytopathogen (+)-Fomannosin

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Total Synthesis of the Phytopathogen (+)-Fomannosin TOTAL SYNTHESIS OF THE PHYTOPATHOGEN (+)-FOMANNOSIN DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Xiaowen Peng, M. S. ***** The Ohio State University 2006 Dissertation Committee: Approved by Professor Leo A. Paquette, Advisor Professor David J. Hart _________________________________ Professor T. V. RajanBabu Advisor Graduate Program in Chemistry ABSTRACT Fomannosin (1) is a sesquiterpene metabolite isolated in 1967 from the medium of the still culture of the wood-rotting fungus, Fomes annosus (Kr.) Karst. It was found to be toxic toward 2-year-old pinus tadae seedlings, Chlorella pyrenoidose, and certain bacteria. Fomannosin features a unique highly strained methylenecyclobutene unit and a reactive doubly unsaturated lactone. It is very sensitive toward both acid and base, posing a considerable challenge to synthetic chemists. The enantioselective total synthesis of (+)-fomannosin was completed in 35 steps starting from known tosylate 13. A zirconocene-mediated ring contraction reaction of vinylated furanosides 20 or 21 was utilized to construct the highly substituted cyclobutane 27. The cyclopentene ring was assembled through a ring-closing metathesis reaction. The lactone ring was then installed by a Knoevenagel condensation of thioester 54. Introduction of the cyclopentanone functionality was accomplished through a dihydroxylation, oxidation and SmI2-mediated -deoxygenation reaction sequence to provide lactones 61 and 62. After the PMB protecting group was removed by trifluoroacetic acid under anhydrous conditions, the first dehydration was effected through the formation of a cyclic ii sulfite. The second dehydration was achieved through the elimination of trifluoromethanesulfonic acid. Deprotection of the TBS ether led to the isolation of (+)- fomannosin. iii Dedicated to my mother Runhua Zhang and my wife Ling Chen In memory of my father Xiaoshengmei Peng iv ACKNOWLEDGMENTS I would like to express my sincere gratitude to my advisor, Professor Leo A. Paquette, for his tremendous support, guidance and encouragement throughout my stay in Columbus. His dedication and work ethic have had a great influence on me. Without his help, I would not have the opportunity to come back to school, and completing this project would not have been possible. I wish to thank Professors David J. Hart and T.V. RajanBabu for their willingness to serve on my dissertation committee. I will always be grateful to a large number of Paquette group members, past and present, for their unconditional help, insightful discussions, and friendship. In particular, I would like to thank Drs. Jiyoung Chang, Maosheng Duan, Ho Yin (Bob) Lo, Feng Geng, Christopher Seekamp, José Méndez-Andino, John Hofferberth, Fabrice Gallou and Nicolas Cunière for sharing their experience in the beginning of my graduate studies here, to Drs. Amy Hart, Jizhou Wang and Zuosheng Liu for their suggestions that were crucial to this project., to Yunlong Zhang, Shuzhi Dong, Zhenjiao Tian, Zhimin Du, Dr. Mike Chang, Dr. Ryan Hartung, Dr. Andreas Luxenburger for their friendship that made my lab life enjoyable. v I am thankful to Drs. Jiong Yang and Ho-Jung Kang for their pioneering work on this project. Special thank goes to Dr. Amy Hart and Cate Stewart for proofreading this manuscript, and all the delicious cookies they have brought. I am thankful to Ms. Donna Rothe and Ms. Rebecca Martin for their kindness and assistance during my stay. Finally, I wish to thank my mother Runhua Zhang, my wife Ling Chen, and my sisters for their infinite love and support that made all of the above possible. vi VITA August 3, 1974. .Born – Jiangxi, China 1996. B. S. Chemistry Nankai University 1999. M. S. Chemistry Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences 1999 – 2000. Graduate Fellow The Ohio State University 2000 – 2001. Graduate Research and Teaching Associate The Ohio State University 2001. M. S. Chemistry The Ohio State University 2002 – 2003. Research Associate Néokimia Inc., Sherbrooke, Canada 2003 – 2006. Graduate Teaching Associate The Ohio State University vii PUBLICATIONS 1. Peng, X.; Bondar, D.; Paquette, L. A. “Alkoxide Precoordination to Rhodium Enables Stereodirected Catalytic Hydrogenation of a Dihydrofuranol Precursor of the C29-40 F/G Sector of Pectenotoxin-2” Tetrahedron 2004, 60, 9589. 2. Xu, Q.; Peng, X.; Tian, W. “A New Strategy for Synthesizing the Steroids with Side Chains from Steroidal Sapogenins: Synthesis of the Aglycone of OSW-1 by Using the Intact Skeleton of Diosgenin” Tetrahedron Lett. 2003, 44, 9375. 3. Paquette, L. A.; Peng, X.; Bondar, D. “Pectenotoxin-2 Synthetic Studies.1. Alkoxide Precoordination to [Rh(NBD)(DIPHOS-4)]BF4 Allows Directed Hydrogenation of a 2,3-Dihydrofuran-3-ol without Competing Furan Production” Org. Lett. 2002, 4, 937. 4. Kong, D.; Yu, Y.; Jia, Y.; Peng, X.; Wong, J. T. “Mechanism Study of Haemoglobin Immobilization on Periodated Oxycellulose” Gaofenzi Xuebao 1999, 2, 221. FIELDS OF STUDY Major Field: Chemistry viii TABLE OF CONTENTS P a g e Abstract. ii Dedication. .iv Acknowledgments . v Vita . .vii List of Schemes. .xi List of Figures . xiii List of Tables. .xiv List of Abbreviations . xv Chapter 1. Introduction . 1 1.1. Isolation, Biological Activity, and Structure Determination . 1 1.2. Biosynthetic Study . 2 1.3. Synthetic Studies on Fomannosin: A Literature Review . 3 1.4. Retrosynthetic Analysis . 9 Chapter 2. Construction of the Cyclobutane and Cyclopentanone Rings . 11 2.1. Synthesis of the Ring Contraction Precursor: Vinylated Furanosides . .. .11 2.2. The Zirconocene-mediated Ring Contraction Reaction . 15 ix 2.3. Assembly of the Cyclopentene Ring . 17 Chapter 3. Assembly of the Lactone Unit . .20 3.1. The Intramolecular Horner-Wadsworth-Emmons Reaction Approach . .20 3.2. The Intramolecular Knoevenagel Condensation Strategy . .26 3.2.1. Knoevenagel Condensation with Monoallyl Malonic Ester . 26 3.2.2. Condensation with Ethylsulfanylcarbonyl Acetic Acid . .31 Chapter 4. Completion of the Total Synthesis . 35 4.1. PMB Deprotection and Dehydration . 35 4.2. The Functionalization of the Cyclopentanone Ring . 39 4.3. First Dehydration . .. 41 4.4. Second Dehydration and the End Game. .44 Chapter 5. Experimental Section . 49 Bibliography . 93 Appendix: 1H NMR Spectra . 98 x LIST OF SCHEMES Scheme Page 1.1 The Biosythetic Pathway of Fomannosin . 3 1.2 Matsumoto's Synthesis of the Fomannosin Skeleton . 5 1.3 Kasugi and Uda's Synthesis of (±)-5,6-Fomannosin Acetate . 7 1.4 Semmelhack's Total Synthesis of (±)-Fomannosin . 8 1.5 Retrosynthetic Analysis of Fomannosin . 10 2.1 Synthesis of 16. 12 2.2 Synthesis of Vinylated furanosides 20 and 21. 13 2.3 Synthesis of Vinylated furanosides 20 and 26. 15 2.4 Zirconocene-mediated Ring Contraction Reaction . 16 2.5 Transition States for Zirconocene-mediated Ring Contraction . 17 2.6 Assembly of the Cyclopentene Ring . 19 3.1 The Cascade Michael Addition - Intramolecular Horner-Wadsworth- Emmons Reaction Strategy . 22 3.2 Preparation of Fragments 36 and 39 . 23 3.3 Initial Attempts to Access 35 . 24 xi 3.4 Attempted Michael addition - intramolecular Horner-Wadsworth- Emmons Cascade Reaction . 25 3.5 Intramolecular Knoevenagel Condensation Strategy. 26 3.6 Preparation of Monoallyl Malonic Acid 44 . .27 3.7 Knoevenagel Condensation of Monoallyl Malonic Ester 45. 28 3.8 Preparation of the PMP Acetal 50 and its Allyl Deprotection. 30 3.9 Attempts to Transform Allyl Ester 50 into Acid Chloride or Thioester 53. .31 3.10 Reduction of Thioester 56. .32 3.11 Formation of Enol 58. 33 3.12 The Reduction of Enol 58 . 34 4.1 PMB Deprotection . .36 4.2 Attempted Dehydration of 64 . 37 4.3 Second Dehydration on 67 . .38 4.4 Attempted Hydroboration . 39 4.5 Functionization of the Cyclopentene . .40 4.6 First Dehydration . .42 4.7 Preparation of 77 and 78 from 62 . 43 4.8 Total Synthesis of Fomannosin (1) . 45 xii LIST OF FIGURES Figure Page 1 Fomannosin and Its Derivatives . 2 xiii LIST OF TABLES Table Page 1 Comparison of 1H NMR Data for Synthetic and Natural Fomannosin. 47 2 Comparison of 13C NMR Data for Synthetic and Natural Fomannosin. .48 xiv LIST OF ABBREVIATIONS α alpha [α] specific rotation Ac acetyl br broad (IR and NMR) β beta n-Bu normal-butyl t-Bu tert-butyl Bz benzoyl °C degrees Celsius calcd calculated COSY correlation spectroscopy CSA (1S)-(+)-10-camphorsulfonic acid δ chemical shift in parts per million downfield from tetramethylsilane d doublet (spectra); day(s) DBU 1,8-diazabicyclo[5.4.0]undec-7-ene DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone DIBAL-H diisobutylaluminum hydride xv DMAP 4-(N,N-dimethylamino)pyridine DMF N,N-dimethylformamide DMSO dimethylsulfoxide EDCI 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride eq. equivalent Et ethyl γ gamma g gram(s) h hour(s) HMBC heteronuclear multiple bond correlation HMPA hexamethylphosphoramide HRMS high resolution mass spectrometry HSQC heteronuclear single quantum coherence Hz hertz IBX o-iodoxybenzoic acid imid. imidazole IR infrared J coupling constant in Hz (NMR) k kilo KHMDS potassium hexamethyldisilazide L liter(s) LDA lithium diisopropylamide m milli; multiplet (NMR) xvi µ micro M moles per liter Me methyl MHz megahertz min minute(s) mol mole(s) Ms methanesulfonyl MS mass spectrometry; molecular sieves m/z mass to charge ratio (MS) NaHMDS sodium hexamethyldisilazide NMO 4-methylmorpholine N-oxide NMR nuclear magnetic reasonance NOE nuclear Overhauser effect (NMR) NOESY nuclear Overhauser and exchange spectroscopy (NMR) p para Ph phenyl PDC pyridinium dichromate PMB p-methoxybenzyl PMP p-methoxyphenyl ppm parts per million PTSA p-toluenesulfonic acid pyr pyridine q quartet (NMR) xvii Rochelle's salt potassium sodium tartrate rt room temperature s singlet (NMR); second(s) t tertiary (tert) t triplet (NMR) TBAF tetrabutylammonium fluoride TBS t-butyldimethylsilyl Tf.
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